High purity phosphorous acid is useful in the production of phosphorates, in salt or ester form, for use in a variety of water treatments, from desealing to corrosion inhibition.
The preparation of phosphorous acid (H.sub.3 PO.sub.3) by the direct oxidation of elemental phosphorus and dissolution of the resulting oxides in water has been difficult to accomplish selectively because of the variety of oxides of phosphorus that can be formed.
A thermodynamic analysis of the oxidation products resulting from reacting phosphorus with an oxidant indicates that, under no circumstances, would P.sub.4 O.sub.6 be a stable phase. Any P.sub.4 O.sub.6 formed should quantitatively disproportionate into P.sub.4 and P.sub.4 O.sub.10 at higher temperatures. However, if one assumes that P.sub.2 O.sub.3 can be formed, this compound is a stable phase over an approximate temperature range from 1500.degree. to 2100.degree. K., with the fraction of phosphorus initially present reporting as P.sub.2 O.sub.3 depending on the nature of the oxidant. The P.sub.2 O.sub.3 so formed can be dissolved in water to form the phosphorous acid or allowed to dimerize to P.sub.4 O.sub.6 and recovered as such.
Oxidants which may be considered for this reaction are oxygen, carbon dioxide and water. The heat release using pure oxygen in stoichiometric amounts is so great that the reaction temperature will exceed the desired range and some form of cooling is required. In addition, precise metering of the reactants must be effected since a deficiency of O.sub.2 will result in lower oxides of phosphorus while an excess of O.sub.2 will generate higher oxides, notably P.sub.4 O.sub.10. This process has been described in U.S. Pat. No. 3,532,461 and German O.S. No. 3,319,606. Carbon dioxide as an oxidant results in a relatively limited temperature range in which P.sub.2 O.sub.3 is stable and other oxides are present. This process is described in U.S. Pat. No. 3,532,461.